.bg {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -999;
}

.bg .bg-container,
.bg .bg-container .item_bg {
    position: absolute;
    width: 100%;
    height: 100%;
    background-repeat: no-repeat;
    background-size: cover;
}